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DS26C31T: Enable Pins on device

Stefanie Frost
Prodigy 40 points
Part Number: DS26C31T

There is an ongoing debate as to whether or not both ENB pins need to be terminated.  Currently we have 1 of them (ENB') tied to GND but the other one is floating.  According to the device's datasheet, these 2 pins are OR'd together indicating there is no need to tie the other ENB to VCC/GND.  Is this the case or will leaving it disconnected cause potential issues to the device's outputs?

  • 0
    TI__Guru 62920 points
    Hi Stefanie,

    As long as /ENABLE is grounded, the DS26C31T device will be enabled regardless of the logic state at ENABLE. In general it is a best practice not to leave logic inputs floating, though, since this can result in increased power dissipation due to the structure of the input. You can read more about that behavior here:

    www.ti.com/.../scba004d.pdf

    Regards,
    Max
  • 0 in reply to Miguel Robertson
    Prodigy 40 points
    Hi Max,

    Thank you for your quick response time and documentation. I just have one more question that I just noticed on our schematic. The outputs are not terminated in the way that the datasheet suggests. They are almost being treated like single ended nets. The designer put 100 Ohm resistors in series with each output vs using them as a termination resistor between the OUT and /OUT pins. Now, having said these, these nets are on a very small board traveling only maybe 1-2" before reaching the connector that goes to the motor. Is the fact that these are not deferentially terminated, though, a risk to the application? We are seeing motor speed inconsistencies which are being quickly categorized as motor failures. I am curious as to whether or not the lag could be caused by the incorrect design of this driver. What are your thoughts?

    Thank you!

    Stefanie
  • 0 in reply to Stefanie Frost
    TI__Guru 62920 points
    Hi Stefanie,

    My apologies for the delay in responding - I must have forgotten to hit the "Reply" button last week.

    The transceiver does not require differential termination for proper functionality. This is something that would improve signal integrity for longer-reach applications where transmission line effects could be observed (i.e., the cable delay is not significantly smaller than the output transition time); I'd expect it to provide no benefit for a 2" connection.

    I wouldn't think 100-Ohm pull-down loading would cause much of an issue on the outputs, either. You can reference Figures 11 through 14 in the datasheet to see the expected driver output high and low levels versus output current (which scales with output resistance).

    That said, a schematic diagram may be helpful here so I can confirm my understanding of the resistor connection is correct. Something else that would help with the debug would be to monitor both halves of the differential signal being received by the motor both when the system is operating normally as well as when it is showing issues. That way, we could see whether the issues are arising from the communication interface and if so what the nature of the signaling problems may be.

    Regards,
    Max
  • 0 in reply to Miguel Robertson
    Prodigy 40 points

    Hello Max,

    No problem at all.  I myself have been swamped so I understand.  We seem to be having intermittent failures with our design, which is actually a pretty simple one.  I have copied the schematic down below.  One connector is tied to the Electrocraft motor that moves the shaft up and down and the other connector is tied to a control board.  I would expect the short distance on the PCB to be insignificant however I am beginning to wonder if the long cable between it and the controller board may be a factor.  Please take a look and let me know when you think when you have time.  The SEN1 device is a Honeywell optical sensor that detects when the shaft is engaged/disengaged which is displayed by DS3 turning off and on.  If you have any questions about this design please let me know.

    Thank you,

    Stefanie

  • 0 in reply to Stefanie Frost
    TI__Guru 62920 points
    Hi Stefanie,

    Thanks for providing the schematic. (Although, note, I've deleted it off of your post since I did see it was marked as confidential and this is a public forum. If you prefer to continue the discussion privately let me know and I can email you instead.)

    I misunderstood your earlier comments about the 100-Ohm resistances. Since these are connected between the transceiver outputs and the connector, they will cause some loss of signal amplitude (especially if differential termination is implemented at the receive end). For example, if the receiver uses 100-Ohm termination, the two series 100-Ohm resistances (one on each half of the differential pair) would cause a signal loss of 67%. So, if you don't know for sure that the receive side has high-impedance inputs then it would be a good idea to look deeper into this. You could use an oscilloscope to verify the differential signal amplitude at the receiver or you could try bypassing the series resistances and seeing if the performance issues resolve.

    Other than that, no real concerns from me on the usage of this part. If you had any way to probe the communication lines while the problems occur I think we would learn a lot, though.

    Max
  • 0 in reply to Miguel Robertson
    Prodigy 40 points

    Hi Max,

    Yes thank you so much for removing the schematic.  I thought this was a private forum.  Yes please email me, that may be easier going forward.  I will respond to your note here once you email me and we will continue from there.  I appreciate it!

    Thanks,

    Stefanie

  • 0 in reply to Stefanie Frost
    TI__Intellectual 1035 points

    Hi Stefanie, We sent you an email on Wednesday (6/5) but haven't seen a response yet.  No worries if you just haven't gotten a chance to look at it I just want to make sure it isn't stuck in a spam folder somewhere.  Thanks, Sean